I'd disagree with others here. If your packs are both 4.8V 750mA and you connect them with a y-cable it's exactly the same thing as having a sealed 4.8V 1500mA pack..... The y-cable connects all the cells in exactly the same as you COULD inside a sealed pack. The charger won't know and doesn't care, you're the only one that knows it's two seperate packs.

We do it all the time when we discharge onboard a plane! Two smaller packs on a y-cable, equals the same double capacity pack.

I wouldn't do it on two packs of different capacity however.

Yea, you can cut up the y-cable and charge at 9.6V 750mA, but why sacrafice the cable? It'll work exactly the same.